1. Field of the Invention
The present invention relates to a metering method of a metal material in injection molding, and more particularly to a metering method of a metal material when nonferrous metal having a low melting point, such as zinc, magnesium, or alloy thereof, is completely melted to allow injection molding in a liquid phase state.
2. Detailed Description of the Prior Art
Attempts have been made to completely melt nonferrous metal having a low melting point so as to allow injection molding in a liquid phase state. Like in the case of injection molding of a plastic material, the molding method thereof adopts a heating cylinder having inside an injecting screw, which is allowed to rotate and move along the axial direction. A granular metal material supplied from the rear portion of the heating cylinder is heated and melted completely while being transferred toward the head of the heating cylinder by means of rotation of the screw, and after a quantity of the metal material in the liquid phase state is metered in the front chamber of the heating cylinder, the metal material is injected into a mold through the nozzle attached to the tip of the heating cylinder by moving the screw forward.
A problem occurring in case of adopting the foregoing injection molding for the metal material is that the material is neither transferred readily -nor metered in a stable manner by means of rotation of the screw.
A molten plastic material has a high viscosity, and transfer of the molten plastic material by means of rotation of the screw is allowed mainly because a friction coefficient at the interface of the molten plastic material and the screw is smaller than a friction coefficient at the interface of the molten plastic material and the inner wall of the heating cylinder, and therefore, a difference in friction coefficient is produced between the two interfaces.
In contrast, the metal material, when melted completely to the liquid phase state, has such a low viscosity compared with the plastic material that a difference in friction coefficient is hardly produced between the above two interfaces. Hence, a transfer force such as the one produced with the molten plastic material by means of rotation of the screw is not readily produced.
However, a transfer force is produced with the metal material when it is in a solid state and in a high viscous region where the metal material is in a semi-molten state during the melting process. Thus, the metal material can be transferred by means of rotation of the screw up to that region. Nevertheless, as the metal material is further melted, the viscosity drops with an increasing ratio of the liquid phase, and the transfer force produced by the screw grooves between the adjacent screw flights decreases, thereby making it difficult to supply the molten metal material in a stable manner to the front chamber of the heating cylinder by means of rotation of the screw.
Because the molten plastic material has a high viscosity, it is stored in the front chamber of the heating cylinder by means of rotation of the screw, while at the same time, a material pressure that pushes the screw backward is produced as a reaction. By controlling the screw retraction caused by the material pressure, a constant quantity of the molten material can be metered each time.
However, the metal material in the low-viscous liquid phase state cannot produce a pressure high enough to push the screw backward. Thus, the screw retraction by the material pressure hardly occurs, and if the metal material is stored in the front chamber by means of rotation of the screw alone, a quantity thereof undesirably varies, thereby making it impossible to meter a constant quantity each time.
In addition, the metal material has a far larger specific gravity compared with the plastic material, and has a low viscosity and fluidity in the liquid phase state. For this reason, when allowed to stand by stopping rotation of the screw, the metal material in the liquid phase state in the heating cylinder placed in a horizontal position leaks into the semi-molten region in the rear portion through a clearance formed between the screw flights and heating cylinder. Consequently, the metal material accumulated in the front chamber causes a back flow onto the periphery of the head portion of the screw through the opened ring valve, and the quantity thereof is undesirably reduced.
The liquid level in the front chamber is lowered with the decreasing accumulated quantity. For this reason, a gaseous phase (space) that makes the metering unstable is generated at the upper portion of the front chamber. In addition, the leaked liquid phase material increases its viscosity in the semi-molten region as its temperature drops,or turns into solid depending on the heating condition in the semi-molten region, thereby forming weirs in the screw grooves. This poses a problem that the granular material supplied from the supply port provided behind the weir cannot be transferred readily by means of rotation of the screw.
The present invention is devised to solve the above problems raised with injection molding of a metal material in the liquid phase state, and therefore, has an object to provide a novel metering method of a metal material in injection molding, by which the metal material in the liquid phase state can be transferred, metered, and deaerated smoothly at all times by placing a heating cylinder in an inclined position, forcing a screw to retract, etc.
In order to achieve the above and other objects, the present invention is a metering method of a metal material in injection molding adapted to employ a heating cylinder provided with a nozzle at a tip thereof and a supply port at a rear portion thereof and having inside a screw, which is allowed to rotate and move along an axial direction, for melting the metal material in the heating cylinder to be transferred to and metered in a front chamber of the heating cylinder in a liquid phase state, and then injecting the metal material through the nozzle by moving the screw forward, and the method includes: placing the heating cylinder in an inclined position with a head thereof pointing downward, so that the metal material in the liquid phase state flows down into the front chamber due to self-weight; forcing the screw at a forward position after injection to retract to a set position while maintaining an inclination, thereby accumulating by suction a predetermined quantity of a liquid phase material stored primarily on a periphery of a head portion of the screw in the front chamber of the heating cylinder by means of a negative pressure; and stopping and then rotating the screw at a retraction position, thereby metering each time a constant quantity of the liquid phase material in the front chamber.
A sensor for counting the number of revolutions of the screw may be provided so as to control the number of revolutions of the screw to stay at a set number of revolutions by means of the sensor. Further, the screw may include an injecting plunger at the tip thereof, which has substantially a same diameter as a diameter of the front chamber formed in the heating cylinder at a top end portion by reducing a diameter thereof so as to be allowed to fit into the front chamber by moving forward and backward while securing a sliding clearance such that hardly causes a back flow of the liquid phase material in the front chamber.
According to the metering method of the present invention, the metal material in the liquid phase state stored primarily on the periphery of the head portion of the screw is accumulated by suction in the front chamber of the heating cylinder by means of a negative pressure produced when the screw is forced to retract. Hence, the metal material can be transferred to the front chamber more readily and reliably compared with transfer by means of screw grooves formed between the adjacent screw flights.
Also, because the heating cylinder is inclined with its head pointing downward so that the metal material is accumulated in the front chamber, an accumulation quantity does not vary due to a back flow even when the metal material is in the low-viscous liquid phase state. In addition, the metal material in the liquid phase state can be stored primarily, and an accumulation quantity in the front chamber can be compensated by means of subsequent rotation of the screw. Hence, a product made of a metal material with a stable molding state can be obtained even by injection molding of the metal material in the liquid phase state.